Aluminum is generally used as the primary interconnect metal in ICs created from silicon semiconductor material. In the early years of IC manufacture, selected parts of an aluminum interconnect layer in a silicon IC normally made direct contact with the silicon. During IC operation, some of the aluminum often diffused into the region occupied by the silicon, and vice versa. This phenomenon, sometimes referred as "aluminum spiking", led to undesirably early device failure. Accordingly, current silicon ICs typically employ an electrically conductive barrier material between the aluminum and silicon in order to inhibit Al-Si interdiffusion.
A number of refractory metals, metallic mixtures, and metallic compounds have been studied for use as the barrier material. Tungsten, titanium-tungsten (a tungsten-rich mixture), and titanium niatride (a compound) are among the most discussed barrier-material candidates.
For example, in U.S. Pat. No. 4,782,380, Shankar et al utilize either titanium-tungsten or titanium nitride as the barrier material. The starting point in Shankar et al is a structure in which a contact opening extends through an oxide layer down to a monocrystalline silicon ("monosilicon") substrate underlying the oxide. A titanium-tungsten or titanium nitride barrier material layer is formed over the oxide and exposed silicon. A rapid thermal anneal ("RTA") is performed on the resulting structure for 30-60 seconds in a nitrogen-containing ambient at a temperature in the range of 500.degree.-800.degree. C. A layer of aluminum or an alloy of aluminum is then deposited over the barrier material layer.
Turning back to the RTA, performance of the RTA in a nitrogen-containing atmosphere enables a nitride-rich skin to form on the exposed surface of the barrier material. This improves the diffusion barrier. A titanium silicide layer forms along the silicon/barrier material interface during the RTA in certain of the embodiments. Although not discussed in Shankar et al, the nitrogen also appears to be responsible for an action that inhibits undesired lateral growth of the titanium silicide. Vadjikar et al, "The Effect of Processing Environment on the Lateral Growth of Titanium Silicide," J. Electrochem Soc., Oct. 1988, pp. 2582-2586, discuss this phenomenon in more detail.
In several titanium nitride embodiments of Shankar et al, an elemental titanium contact layer is deposited on the structure prior to titanium-nitride formation. A very similar technique, including an RTA after titanium nitride barrier material deposition, is employed by Asahina in published Japanese patent application 63-84024. It appears that all of the titanium is converted into titanium nitride or titanium silicide in both Asahina and Shankar et al.
There are major disadvantages with the preceding interconnect formation techniques. The present inventors investigated the titanium-tungsten technique and found that the resulting contact resistance to heavily doped P-type monosilicon was higher than that for a simple aluminum contact to heavily doped P-type monosilicon. This is highly undesirable. As to the titanium-nitride techniques, preparation of reproducible well-controlled titanum nitride layers poses severe processing difficulties.
Shishino, published Japanese patent application 62-213159, also describes an interconnect fabrication technique in which a titanium contact layer is created prior to barrier material formation. The titanium contact layer is deposited on an oxide layer and into a contact opening down to a P-type or N-type zone of an underlying monosilicon substrate. A titanium-tungsten barrier material layer is deposited on the Ti layer. After an aluminum layer is deposited on the titanium-tunsten, selected portions of the three-layer stack are removed to create a desired interconnect pattern. The resulting structure is then annealed at 450.degree. C. in a nitrogen-containing ambient.
Shishino claims that his process results in lowered contact resistance and good barrier material characteristics. While the contact resistance in Shishino may actually be reduced, the present inventors' investigation of Shishino's work indicates that his contact resistance may not be low enough for future long-life applications.